Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization or particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g. EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP-625,728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer particle (latex), as described in e.g. EP-As 770 494, 770 495, 770 496 and 770 497. These patents disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, (2) and developing the image-wise exposed element by applying fountain and/or ink.
Some of these thermal processes enable plate-making without wet processing and are for example based on ablation of one or more layers of the coating. At the exposed areas the surface of an underlying layer is revealed which has a different affinity towards ink or fountain than the surface of the unexposed coating.
Other thermal processes which enable plate-making without wet processing are for example processes based on a heat-induced hydrophilic/oleophilic conversion of one or more layers of the coating so that at exposed areas a different affinity towards ink or fountain is created than at the surface of the unexposed coating.
U.S. Pat. No. 4,576,743 discloses a plate cleaner comprising an aqueous solution containing a silicate and at least one is surface active agent selected from a cationic or an amphoteric surface active agent.
A stable composition comprising an alkaline component in combination with a chemical compound that reduces or prevents residue and scum formation such as aromatic sulfur containing compounds and sugars and sugar derivatives is disclosed in EP 1 361 480.
A cleaning composition for lithographic printing plates comprising (i) mainly aliphatic hydrocarbons with a specific boiling point and flash point, (ii) a surfactant with a hydrophilic/lipophilic balance of about 3 to 10 and (iii) an electrolyte selected from a silicate, sulfate, phosfate or nitrate salt is disclosed in U.S. Pat. No. 4,504,406.
U.S. Pat. No. 5,691,288 discloses a composition consisting essentially of a stable emulsion comprising 0.1% wt to 7% wt polyol, 1% wt to 15% wt starches or dextrins, 0.5% wt to 2% wt alkyl benzene sulfonate amine salt and 1.0% wt to 20% wt hydrocarbons containing less than 10% wt aromatic hydrocarbons, 0.1 to 5% wt substituted phenoxypoly(oxythelene) ethanol, about 0.1% wt C12 to C20 alcohol and 0.01% wt to 1.0% wt ethanol amine.
U.S. Pat. No. 4,829,897 discloses a blanket washing medium comprising 5% wt-35% wt water and 65% wt-95% wt of a water insoluble phase that contains certain hydrocarbons in a specific proportion and a surfactant with a HLB value of 3-11.
A cleaning composition for removing inks and/or oil residues located on a surface of a printing apparatus comprising an alkyl (poly)glucoside is disclosed in U.S. Pat. No. 6,346,156.
WO 95/14755 discloses a cleaning composition comprising a mixture of a hydrocarbon solvent, a low VOC C12-C26 alkyl benzene wherein the alkyl group is a C6-C20 alkyl group, a low VOC C16-C30 alkyl naphtalene wherein the alkyl group is a C6-C20 alkyl and optionally a surfactant.
Before, during and after the printing step, a lithographic printing plate is in general treated with various liquids for improving the lithographic properties of the image and non-image areas. Such liquids are applied for example to improve the hydrophilic properties of the non-image areas and to protect, restore or even enhance the hydrophobicity of the image areas. It is of high importance that these fluids, commonly referred to as plate treating liquids, do not deteriorate the image and/or the non-image areas throughout and well after their application. Due to the bivalent nature of such a treatment—i.e. improving both hydrophilic and hydrophobic areas—the treating liquid typically contains both water and organic solvent(s) and is thus an emulsion; preferably an oil-in-water (O/W) emulsion.
The cleaning strength or so-called “ink solvency” of plate cleaning liquids or plate cleaners—i.e. the ability to remove ink from a plate—is mainly determined by the composition of the plate cleaner and more specificly, by the concentration and/or nature of the organic solvent. Aromatic hydrocarbon solvents are preferred over aliphatic hydrocarbon solvents as they exhibit a very good ink solvency. Commonly used aromatic hydrocarbon solvents are mixtures of C9-C10 alkylbenzene hydrocarbons. However, cleaning liquids containing such aromatic hydrocarbons have a low flashpoint and thus create a high risk of explosure to the cleaning liquids not only in the pressroom environment but also during transport. Aromatic hydrocarbons with higher flashpoints such as for example mixtures of C10-C11 alkylbenzene hydrocarbon solvents are preferably not used as they have a reduced ink solvency and they usually contain naphthalene and/or naphthalene derivatives. The presence of naphtalene and/or naphthalene derivatives in cleaning liquids should be limited as these compounds have a nasty smell and are classified as carcinogenic compounds; for example they are classified as category 2B carcinogens by the International Agency for Research on Cancer (IARC 2002). The flashpoint of C9-C10 aromatic alkylbenzene hydrocarbon solvents can for example be increased by mixing them with aliphatic hydrocarbon solvents with a higher flashpoint. However, the solvent level of such a plate cleaner significantly increases which is unfavourable from both an environmental and economic point of view.
Therefore, there is still an urgent need for efficient treating liquids which meet high standards of health and safety.